This paper describes the diel periodicity of feeding activities and stomach contents compositions of two young flounders, Pleuronichthys cornutus and Pleuronectes yokohamae which show a sympatric distribution in Suo-nada, western Seto Inland Sea. Sampling by a small trawl net was conducted 11 times over a period of 24 hours during 7 and 8, July, 1984. The number of caught flounders, P. cornutus and P, yokohamae were 463 (Body length, 50∼110 mm) and 602 (Body length, 30∼90 mm), respectively. Both species showed a similar die! feeding activity. The stomach contents weight index (SCI=stomach contents weight×10
2/body weight) increased from early morning to about sunset. After sunset, the SCI began to decrease and reached the minimum before sunrise. The highest SCI of P. cornutus was approximately 1.5, which was three times larger than that (nearly 0.5) of P, yokohamae. Moreover, the percentage of fish with empty stomach was high just before sunrise and became very low during daytime. These results show both species feed on prey chiefly from early morning to evening. The stomach contents of P. cornutus and P. yokohamae consisted mostly of small size polychaetes, Paraprionospio sp. (Type B) and Lumbrineris sp., respectively, both of which were dominant in the macrobenthic fauna in Suo-nada. The continuous observations on the stomach contents of both flounders clearly showed that Paraprionospio sp. (Type B) tends to be digested more easily than Lumbrineris sp. This fact strongly suggests that P, cornutus has an advantage on the feeding strategy over the sympatric counterpart.
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